1. Technical Field of the Invention
The present invention generally relates to catalysts, processes and apparatus for producing synthesis gas from light hydrocarbons. More particularly, the invention pertains to catalysts that are active for catalyzing the selective partial oxidation of light hydrocarbons (e.g., methane or natural gas) to products containing CO and H2 and the concurrent catalytic partial oxidation of H2S to elemental sulfur and water, and to methods and apparatus employing such catalysts for enhancement of syngas production.
2. Description of Related Art
Many refineries face an abundant supply of lower alkanes, i.e., C1–C4 alkanes such as methane, and relatively few means of converting them to more valuable products. Moreover, vast reserves of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. There is great incentive to exploit these natural gas formations, however most natural gas formations are situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported than syngas.
The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes. Syngas generation from methane typically takes place by one of three techniques. Steam reforming of methane is the most common, followed by partial oxidation, and autothermal reforming.
The partial oxidation of methane can be represented by the reaction shown in equation (1):CH4+½O2→CO+2H2  (1)At the same time, some of the methane burns completely, according to equation (2):CH4+2O2→CO2+2H2O  (2)
Hence, syngas is typically a mixture of carbon monoxide and molecular hydrogen, generally having a hydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and may contain other gases such as carbon dioxide. Synthesis gas is not usually considered a commodity; instead, it is typically generated on-site for further processing. Synthesis gas is commonly used as a feedstock for conversion to alcohols (e.g., methanol), olefins, or saturated hydrocarbons (paraffins) according to the well-known Fischer-Tropsch process, and by other means. The resulting high molecular weight (e.g. C50+) paraffins, in turn, provide an ideal feedstock for hydrocracking, a feedstock for conversion to high quality jet fuel, and superior high octane value diesel fuel blending components.
Emerging technologies that have been developed to generate syngas from methane include a technique that entails exposing a mixture of methane and oxygen to a hot catalyst for a brief time, typically on the order of milliseconds, followed by cooling of the resultant gas stream. EPO Patent No. 303,438 describes a process for synthesis gas production by catalytic partial oxidation to overcome some of the disadvantages and costs of steam reforming. A monolith catalyst is used with or without metal addition to the surface of the monolith and the process operates at space velocities of 20,000–500,000 hr−1. Conventional catalytic partial oxidation processes are also described, for example, in U.S. Pat. Nos. 5,654,491, 5,639,929, 5,648,582 and in J. Catalysis 138, 267–282 (1992), the disclosures of which are incorporated herein by reference. Although in conventional short contact time syngas generation systems the syngas reaction can be self-sustaining once initiated, it has been shown that 10–15% of the carbon initially present as methane can be lost to the formation of CO2 in combustion via equation (2), above. This directly reduces the yield of syngas that can be obtained. Therefore it is desirable to use a syngas generation system that allows a better yield of carbon monoxide and hydrogen.
Further complicating the exploitation of the world's natural gas supply is the fact that many natural gas formations contain H2S in concentrations ranging from trace amounts up to about 3–25% (by volume) hydrogen sulfide. For example, many of the catalysts that are conventionally used for the production of synthesis gas are poisoned by the presence of sulfur.
If the hydrocarbon conversion does proceed to some degree, the syngas product is typically contaminated by passed through H2S and/or SO2. The presence of H2S or SO2 generally diminishes the usefulness of the syngas or creates environmental safety concerns. It would be highly desirable in the natural gas exploitation industry to find a way to efficiently convert the light hydrocarbon content of the natural gas to synthesis gas without conducting an initial sulfur removal operation. In a related aspect of petroleum refining, some petroleum feed streams and separated fractions contain sulfur. Sulfur is typically undesirable in most petroleum refining processes and products. Refineries typically upgrade the quality of the various petroleum fractions by removing the sulfur before they are processed further. Hydrodesulfurization units are used to break down the sulfur compounds in the petroleum fractions and convert the sulfur to H2S. In addition to hydrodesulfurization processes, other conversion processes in a typical refinery, such as fluid catalytic cracking, coking, visbreaking, and thermal cracking, produce H2S from sulfur containing petroleum fractions. The H2S from both the desulfurization processes and these conversion processes is typically removed from the gas streams or light liquid hydrocarbon streams using either chemical solvents based on alkanolamine chemistry or physical solvents. A circulating, regenerative H2S removal system employing an absorption stage for H2S pickup and a regeneration stage for H2S rejection produces a concentrated stream of H2S.
In conventional systems, this H2S stream is then fed to a H2S conversion unit, which converts the H2S into a storable, saleable product such as elemental sulfur, sodium hydrosulfide solution, or sulfuric acid. Conversion of the H2S to elemental sulfur is most common, mainly because elemental sulfur is the most marketable sulfur compound of those typically produced.
The process most commonly used to recover elemental sulfur from H2S gas is the modified Claus sulfur recovery process. The conventional Claus process is well known in the art, and is also described in U.S. patent application Ser. No. 09/624,715, now U.S. Pat. No. 6,403,051 the disclosure of which is incorporated herein by reference.
U.S. Pat. No. 5,720,901 describes a process for the catalytic partial oxidation of hydrocarbons in which nitrogen is present in the hydrocarbon feed mixture. An organic or inorganic sulfur-containing compound is present in the feed mixture in a sufficient concentration (i.e., 0.05 to 100 ppm) to reduce the presence of nitrogen by-products, particularly ammonia and hydrogen cyanide, in the products of the catalytic partial oxidation process. It is suggested that hydrocarbon feedstocks used directly from naturally occurring reservoirs in which the sulfur content is significantly above the stated low levels be subjected to a partial sulfur removal treatment before being employed in that process. A sulfur removal step is applied to the product stream if the carbon monoxide and/or hydrogen products of the process are to be utilized in applications that are sensitive to the presence of sulfur, such as Fischer-Tropsch synthesis.
What is needed is a syngas production process than can avoid the need for an initial sulfur-removal step from H2S-containing natural gas sources. The industry would also welcome a CPOX-based syngas production process that can avoid the undesirable side reaction by which a small but significant amount of the hydrocarbon is converted to CO2, so that the yield and selectivity for CO and H2 products could be improved. Also needed are new and better ways to utilize H2S gas streams arising from existing desulfurization processes.